Recently, energy saved electrophotographic image forming apparatus is required to in view of countermeasure of global environment contamination, in particular, image forming apparatus energy saving of fixing system, which consumes a plenty amount of energy among the electrophotographic image forming apparatus, is required.
Hitherto, a resin having sharp melt property, specifically crystalline polyester resin, is known to use in a toner as a binder resin for one of an effective method for low temperature fixing. Further, a method using a polyester resin having a crosslinked structure having excellent high temperature elasticity is known to solve problems such as anti-high temperature offset property or anti-heat durable property caused by crystalline polyester resin (see, for example, patent document 1).
However, it is difficult to produce a toner having a small particle diameter containing a polyester resin having a crosslinked structure by above described toner producing method, and therefore, an image having high quality is difficult to form.
On the other side, it is known that a toner having a small particle diameter can be produced by employing polymerization method.
However, it, is difficult to produce a toner containing a polyester resin having a crosslinked structure.
The polyester resin is formed by a dehydration condensation reaction, and it is difficult to progress in water. For the countermeasure, it is proposed a method wherein the polyester resin formed by dehydration condensation reaction is dissolved in organic solvent preliminary, it is dispersed in an aqueous medium to obtain emulsification liquid via phase reversal of emulsion emulsification method or the like, and toner particles are formed using this. However there is a problem in this method because the polyester resin having a crosslinked structure is difficult to be dissolved or dispersed. In organic solvent and needs plenty of energy to dissolve or disperse.
For solving the problem, a method is provided that crosslinked structure is formed when particles are formed employing a polyester resin to which isocyanate group is introduced, (see, for example, patent document 1).
However, there is a problem that it is difficult to control reaction and impossible to produce stably by the toner producing method because of using isocyanate group having extremely high reactivity.